Pneumocystis carinii is a parasitic microorganism which induces an often fatal pneumonitis in a variety of compromised patients (e.g., premature infants, those with congenital immune deficiency disorders, those therapeutically immune suppressed, etc.). Organisms derived from murine sources were cultivated in vitro on monolayers of primary embryonic chick epithelial lung cells. The infected cultures were then examined by scanning electron microscopy. An examination of the surface ultrastructure revealed pleomorphic organisms which were not only attached to host cells with anchoring fibers but also connected to other parasites via fine fibrils. Another type of fine fibril was observed which may play a role in the organism's nutrition.